Chloroplasts and mitochondria of eukaryotic cells possess their own distinctive DNA and protein synthesizing systems. Although essential for organelle function, organelle genomes contain sufficient information to specify only certain organelle components, leaving the nuclear genome to code for most proteins found in both chloroplasts and mitochondria. We are just beginning to understand the full extent of the cooperativity between the nuclear and organelle genomes in coding for the many components making up a mitochondrial cristae and the chloroplast thylakoid membrane as well as the ribosomes of both organelles. This cooperativity appears to extend even to the level of individual macromolecules such as the CO2 fixing enzyme ribulose bisphosphate carboxylase in the chloroplast, cytochrome oxidase in the mitochondria, and the F1 ATPase in both chloroplast and mitochondria. The unicellular green alga Chlamydomonas reinhardtii is the only organism in which chloroplast genes can be mapped both genetically and physically, and in which both chloroplast and mitochondrial genomes can be probed using genetic as well as molecular methods. Our research efforts center on the interplay between organelle and nuclear genes in this alga. The major objectives, as stated in our five-year proposal, are to understand the interaction of chloroplast and nuclear genes in the biogenesis of organelle ribosomes and to isolate, characterize, and map both physically and genetically mutations in organelle DNA affecting different organelle functions.